2023 AIME II Problems/Problem 11
Problem
Find the number of collections of distinct subsets of
with the property that for any two subsets
and
in the collection,
Solution
Denote by a collection of 16 distinct subsets of
.
Denote
.
Case 1: .
This entails .
Hence, for any other set
, we have
. This is infeasible.
Case 2: .
Let .
To get
for all
.
We must have
.
The total number of subsets of that contain
is
.
Because
contains 16 subsets.
We must have
.
Therefore, for any
, we must have
.
So this is feasible.
Now, we count the number of in this case.
We only need to determine
.
Therefore, the number of solutions is 5.
Case 3: .
Case 3.1: There is exactly one subset in that contains 2 elements.
Denote this subset as .
We then put all subsets of
that contain at least three elements into
, except
.
This satisfies
for any
.
Now, we count the number of in this case.
We only need to determine
.
Therefore, the number of solutions is
.
Case 3.2: There are exactly two subsets in that contain 2 elements.
They must take the form and
.
We then put all subsets of that contain at least three elements into
, except
and
.
This satisfies
for any
.
Now, we count the number of in this case.
We only need to determine
and
.
Therefore, the number of solutions is
.
Case 3.3: There are exactly three subsets in that contain 2 elements.
They take the form
,
,
.
We then put all subsets of that contain at least three elements into
, except
,
,
.
This satisfies
for any
.
Now, we count the number of in this case.
We only need to determine
,
,
.
Therefore, the number of solutions is
.
Case 3.4: There are exactly three subsets in that contain 2 elements.
They take the form
,
,
.
We then put all subsets of that contain at least three elements into
, except
,
,
.
This satisfies
for any
.
Now, we count the number of in this case.
We only need to determine
,
,
.
Therefore, the number of solutions is
.
Case 3.5: There are exactly four subsets in that contain 2 elements.
They take the form
,
,
,
.
We then put all subsets of that contain at least three elements into
, except
,
,
,
.
This satisfies
for any
.
Now, we count the number of in this case.
We only need to determine
,
,
,
.
Therefore, the number of solutions is 5.
Putting all subcases together, the number of solutions is this case is .
Case 4: .
The number of subsets of that contain at least three elements is
.
Because
has 16 elements, we must select all such subsets into
.
Therefore, the number of solutions in this case is 1.
Putting all cases together, the total number of is
.
Solution 2
Denote the as
and the collection of subsets as
.
Case 1: There are only sets of size or higher in
:
Any two sets in
must have at least one element common to both of them (since
). Since there are
subsets of
that have size
or higher, there is only one possibility for this case.
Case 2: There are only sets of size or higher in
:
Firstly, there cannot be a no size
set
, or it will be overcounting the first case.
If there is only one such size set, there are
ways to choose it. That size
set, say
, cannot be in
with
(a three element set). Thus, there are only
possible size
subsets that can be in
, giving us
for this case.
If there are two sets with size in
, we can choose the common elements of these two subsets in
ways, giving us a total of
.
If there are three sets with size , they can either share one common element, which can be done in
ways, or they can share pairwise common elements (sort of like a cycle), which can be done
ways. In total, we have
possibilities.
If there are four sets with size , they all have to share one common element, which can be done in
ways.
Thus, summing everything up, this will give us possible sets
Case 3: There is a set with size in
:
Notice that be at most one size
subset. There are
ways to choose that single element set. Say it's
. All other subsets in
must have a
in them, but there are only
of them. Thus this case yields
possibilities.
Thus, the total number of sets would be
.
~sml1809
Video Solution
See also
2023 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 10 |
Followed by Problem 12 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
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